rubocop noise: fixes FrozenStringLiteralComment

Adding the .ruby-verison file triggered previously un-run cops, specifically:

  This cop is designed to help upgrade to Ruby 3.0. It will add the
  comment `# frozen_string_literal: true` to the top of files to enable
  frozen string literals. Frozen string literals will be default in Ruby
  3.0. The comment will be added below a shebang and encoding comment. The
  frozen string literal comment is only valid in Ruby 2.3+.

More info on rubocop [Automatic-Corrections](https://github.com/bbatsov/rubocop/wiki/Automatic-Corrections)

app/mailers/application_mailer.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 class ApplicationMailer < ActionMailer::Base
   default from: ENV['default_mail_from']
   layout 'mailer'

app/mailers/candidate_mailer.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 class CandidateMailer < ApplicationMailer
   def welcome candidate
     @candidate = candidate

app/mailers/recruiter_mailer.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 class RecruiterMailer < ApplicationMailer
   def candidate_created candidate
     @candidate = candidate

app/mailers/reviewer_mailer.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 class ReviewerMailer < ApplicationMailer
   def candidate_submission candidate
     @candidate = candidate

app/mailers/user_mailer.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 class UserMailer < ApplicationMailer
   def password_reset user
     @user = user